Season 1

Natsuki has been acting strange and seems to have his head in the clouds through everything he does. After being interrogated by Minamo and the others, he reluctantly confesses to them that he kissed Atri in the ocean.

Was Natsuki's first love, Atri? His beautiful childhood memories are gradually being overwritten. Despite his unwillingness to admit it was her the facts around him are lining up to force him to, but he continues to deny it.

Natsuki's life experiences a big shift after becoming a teacher and starting a salvaging service. Rumors of a school lit with electricity attracts new students, and activities around the school regain a sense of liveliness.

If Natsuki sells Atri, he can get batteries. However, he can no longer think of Atri as just an average humanoid. While he's deep in thought on the dock, looking out at the ocean, Natsuki finds Catherine floating in the water. She is completely intoxicated, and Natsuki and Atri take her in. But right at that time, the sound of a gunshot echoes within the silent ship.

Natsuki sets his heart on creating a tidal power generator that utilizes the ebb and flow of the ocean's tides. He dives back into the ocean to find some materials and salvages some resin from the ruins of Ryuuji's family's factory. As soon as he gets back he gets to work. As they are working, Natsuki and Ryuuji exchange heated opinions over how the generator should be built. Atri intervenes and the two guys become closer friends. Meanwhile, a suspicious-looking character peeks at them from outside of the school....

Natsuki accompanies Atri who is on her way to school. However, when they arrive, they find that most of the school is underwater. Although they managed to get into the school building, inside it was full of empty classrooms and an unexpected teacher. There, Natsuki is faced with the saddening reality of the lives of the people who remained in their town.

Atri struggles to fulfill her duties after falling into the ocean and failing at cooking. She tries to recall an important order from Nonko but her memories seem faulty.

In the near future, much of the world has been forgotten at the bottom of the ocean due to rising sea levels. Natsuki Ikaruga, who lives in a small town surrounded and stranded by the sea, is invited by a mysterious woman, Catherine, to search for a treasure in his grandmother's house who was a marine geologist. One day, he dives to the bottom of the ocean in a submarine and encounters a mysterious capsule with a girl sleeping inside. The girl was a humanoid so sophisticated that she could be mistaken for a human.